Coalescing binary black holes originating from globular clusters

Dorota Gondek-Rosinska University of Zielona Gora A. Askar, M. Szkudlarek,D.Gondek-Rosinska, M.Giersz,T.Bulik, 2017,MNRAS

The recent breakthroughs

● 2015 - detection of gravitational waves by aLIGO → GW Astronomy, a new window onto the Universe

● Detection of coalescing black hole binaries: GW150914,GW151226, GW170104,GW170608,GW170814 and LVT151012 ● Observation evidence that BBHs merge within Hubble time

● Evidence for massive stellar BHs with masses of 30 and up to 60 solar masses (their formation requires an origin from low metalicity environments (Belczynski et al. 2010, 2016))

● GW150914 - the “brightest” source ever observed

Expect a lot of discoveries in near future by Advanced LIGO/VIRGO detectors !!!

Where does it fit into broad astrophysical picture? -evolution of binaries in the field (Belczynski et al. 2016) -formation of binaries in dense clusters -population III

Globular Clusters

Spherical collections of stars that orbits a galactic core as a satellite. More than 1000 extragalactic GC (HST) up to 375 Mpc. ~157 GC in Milky Way (Harris catalog)

GC contain 10000 to several milions stars

Most of stars are old Population II (metal-poor) stars

Stars are clumped closely together, especially near the centre of the cluster --> close dynamical interactions → tight binary systems containing compact objects

Globular Clusters in the Milky Way are estimated to be at least 10 billion years old. 50% GC within 5kpc, the most distant 130 kpc

Credit: M. Benacquista & Downing, 2011, the distribution of 157 GC in the Milky Way from Hariss catalog

Code description

● We use the MOCCA (MOnte Carlo Cluster simulAtor) code developed by Mirek Giersz, Henon (1971), Stodolkiewicz (1982), Abbas et al. (2016, 2017). Similar to the code used by the Northwestern group (Rodriguez et al.) ● Well tested, allows to investigate individual interactions, while ensuring that the evolution of cluster is accurate and computationally efficient. ● BIGSURVEY – 2000 MOCCA models, range of metallicities and sizes to match the population of GCs in the Milky Way ● Matches Milky Way but is not a fit. Many degeneracies.

Summary of simulations Metallicity Total mass Mass range Number of Number of of clusters models BHBH [106 Msun] [106 Msun] mergers 0.02 51.7 0.024-0.61 258 735

0.006 19.6 0.63 31 1857

0.005 49.4 0.024-0.61 243 3042

0.001 141 0.02-1.08 423 9169

0.0002 18.9 0.63 30 2276

Merging BBHs and Colliding BHs From Globular Clusters

Number of merging BH binaries or colliding BH within Hubble time per unit time (1 Myr) as a function of merger time for black holes. Five different interactions, which can lead to the emission of chirp GW signal (dashed lines) due to the coalescence of two BHs in a binary system or a burst GW signal (solid lines) due to the collision of two BHs.

● Merger of BBH – a chirp signal - EBE -ejected binary evolution - RBE – retained binary evolution

● Colliding 2 BHs – a burst signal due to dynamical 2-body,3-body or 4-body interactions

BBH Mergers due GW radiation from Globular Clusters

Number of merging BBH binaries within Hubble time per unit time (1 Myr) as a function of merger time for black holes with MBH < 100Msun BBH in GC: 3 000; BBH ejected from GC ~15 000,

● Path to BBH merger - escaping binaries (dominating) -binary evolution inside GC

● Mass distribution? ● BBH production efficiency ?

Dependence on the cluster mass

BBH production efficiency:GC vs Field Number of merging BBH binaries per 10^6 solar masses of stars. Field data from Belczynski et al 2016

Local merger rate density for BBH merger The dominant contribution – escaping BHBH

Merger rates in clusters

● Globular Cluster formation rate

Katz & Ricotti 2013

0 2 4 6 8 ● GC mass composition

● GC metallicity

● The local merger rate (Abbas,Szkudlarek, Rosinska, Bulik, Giersz 2017) - 5.4 Gpc^-3/yr - 30 Gpc^-3/yr if we include GC with 10^7 Msol,

● Systematic uncertainties to be understood

Field vs Globular Clusters

● Can we use spins to distinguish the two? ● GC formation – exchanges, non aligned spins

● Are spins aligned in field evolution?

● Can we use eccentricities to distinguish the two?

● In the field only 0.1% with e > 0.01 (Kowalska et al. 2011)

● In GC, dynamically-formed binaries highly eccentric ?

Eccentricity of BBH at ejection

Eccentricities of coalescing BBH at 10 Hz ...but 3-body interactions

Globular clusters and gravitational waves

Work in progress

30 % of globular cluster models contain IMBHs, 100-10000Msol (Giersz et al. 2015). One of formation scenario: built up BH mass due to mergers in dynamical interactions and mass transfer in binaries

Summary

● We have explored mergers of BBHs from 1000 GC using well tested MOCCA code.

● The dominant contribution is from ejected BBH and low metalicity models ● The local merger rate density of BBH from globular cluster for LIGO/VIGO detectors (masses of BH < 100 Msol) is 5.4-30 Gpc^-3/yr (Abbas,Szkudlarek,Rosinska,Bulik,Giersz 2017)

● Rates are in the low end of the observed values – Depends on assumptions on cluster mass and metallicity distribution ● Mass distribution of BBH consistent with aLIGO/Virgo observations -Predict a tail of higher mass object merging inside clusters

● The number of eccentric BBH systems ejected from clusters or merged in GC will not be a significant source for Advanced LIGO/Virgo (..but BH in triple systems etc) ● The IMBH (> 100 Msol) is formed in 30 % GC models → many BH-BH collisions ● Expect a lot of discoveries in near future !!!

Eccentricities of coalescing BBH at 10 Hz ...but 3-body interactions

Model vs Milky Way Globular Clusters

Stellar dynamics and Globular Clusters

Local Merger Rate Density of BBH Mergers